Evaluating the influence of weathering on erodibility and cross-channel geometry in bedrock channels

Erosion of bedrock-floored channels regulates the evolution of many landscapes, and accurate prediction of channel erosion rates is a primary goal of landscape evolution modeling. Field and modeling studies suggest that bedrock channels equilibrate through adjustment of geometry and slope to the imposed discharge, sediment supply, and substrate erodibility conditions. In numerical models, substrate erodibility is typically considered a constant within a channel cross section. However, our study demonstrates that weathering produces differences in rock erodibility across some channels. These spatial variations in erodibility may be an important control on channel geometry and gradient. We use field studies, laboratory experiments, and numerical modeling to assess the influence of weathering on erosion of bedrock-floored stream channels. Field data have been collected from stream channels with exposed sandstone, basalt, granite, and limestone bedrock. Schmidt hammer measurements indicate that the compressive strength of bedrock exposed along channel margins differs significantly from rocks found at the center of the channel in some locations. Field observations of weathering are consistent with these measurements. We quarried bedrock samples from channels floored by sandstone and basalt. The preserved surfaces of these samples were subjected to abrasion by 4-8 mm gravel in laboratory erosion mills. Results from abrasion mill experiments are consistent across sites: (1) abrasion rate decreases with time, as weathered material is removed; (2) channel margin samples abrade 3-5 times faster than channel center samples; and (3) effects of weathering on abrasion extend to a depth of ~1 mm. Using the strategy of Wobus et al. (2006), we model the influence of weathering on channel geometry and slope as mean peak discharge (Qm) and erosion rate (E) vary. The ratio of erosion rate to weathering rate is a primary control on the influence of weathering. At low uplift rates, weathering outpaces erosion, and channels are wider and less steep than fixed erodibility channels. At high uplift rates, erosion outpaces weathering and erodibility, and weathering does not influence geometry or gradient. Weathering channels differ most from fixed erodibility channels at intermediate uplift rates, with greater width/depth ratios and lower slopes.